When racing a bicycle, particularly when racing a mountain bike downhill, there are intervals during which the drive chain, particularly along its lower (return) run is under little tension, and subjected to varying high forces and vibration. During these intervals, the chain can whip wildly. All too frequently, if the drive sprocket is cranked while the chain is whipping, the loose chain will become jammed (wedged) between the sprocket and the bicycle frame. This event is sometimes designated to mountain biking enthusiasts by the term "chain suck".
Others have proposed solutions to this problem.
The U.S. Pat. of Nagano, 4,786,071, issued Nov. 22, 1988, discloses a device (perhaps the one sold as the Shimano Sharktooth), which mounts on the chain stay of the frame and has a guard body flange which extends upwards and inwards on a slant to intercept and thereby guide the lower run of the chain from below.
The U.S. Pat. of Klein 4,960,402, issued Oct. 2, 1990, discloses a device, similar to Nagano's and said to be an improvement. It has additional curved flanges 40, 41, 42 which are said to raise the chain out of the chain ring teeth.
It is the present inventor's conclusion from reviewing those two prior art patent documents, that the devices shown in them will not damp undesirable bicycle sprocket drive chain vibration. Rather, these devices attempt to prevent the chain from becoming stuck only after the chain already has been displaced, e.g., by the high forces encountered when cycling downhill off-road at high speed, such as at forty miles per hour.
On a conventional mountain bike, undamped chain motions can be caused by transient waves or inertial displacement, or both, in the vertical plane. Lateral forces usually are relatively low assuming the bike is upright.